As demand for wireless data in cellular networks increases, user expectations of high data rates along with seamless mobility also increases. In order to fulfill the increasing user expectations of high data rates and seamless mobility, more wireless network spectrum may need to be made available for wireless subscribers. Typical Long Term Evolution (LTE) wireless networks are designed to use a dedicated licensed spectrum. The spectrum in which a wireless network operator deploys a LTE network is solely used by that operator to serve that operator's wireless network subscribers. To meet the growing demand for wireless network spectrum, the 3rd Generation Partnership Project (3GPP) has proposed using radio-frequency carriers in the unlicensed spectrum, which is often referred to as “Long Term Evolution (LTE) in Unlicensed” (“LTE-U”) (also referred to as Licensed-Assisted Access (LAA)). Generally, the unlicensed spectrum includes frequencies in the 5 GHz band.
The basic premise of LTE-U is to extend the LTE protocols into the unlicensed spectrum in order to supplement data traffic in the licensed spectrum. Because wireless network operators deploy their LTE systems in their corresponding licensed spectrum, data transmission and related acknowledgement feedback do not typically require any contention mechanisms. However, since the unlicensed spectrum can be shared by more than one wireless network operator, as well as other radio access technologies (for example, Wi-Fi), sharing and contention mechanisms may be required to deploy LTE in the unlicensed spectrum.
When LTE-U is used as a supplemental carrier, the aforementioned contention mechanisms can be employed to transmit downlink and/or uplink data using the unlicensed spectrum. Even though data is transmitted using the unlicensed spectrum, it is more likely that the transmission of more delay and quality of service (QoS) sensitive service (for example, hybrid automatic repeat request (HARQ)-acknowledgement (ACK) feedback), will likely be carried out using the licensed spectrum because the licensed spectrum may be considered more reliable than the unlicensed spectrum. However, as the use of the unlicensed spectrum increases, the transmission of HARQ-ACK feedback over licensed channel will likely induce significant overheads. Therefore, the use of unlicensed band for HARQ-ACK feedback transmission may be advantageous.
Moreover, several scenarios can be envisioned for the future 5G technology, where LTE will be deployed as a Stand-Alone (SA) carrier in the unlicensed band. Therefore, new mechanisms may be required to transmit HARQ-ACK information in the unlicensed band.